@advocatux the last section of the file http://system-image.ubports.com/ubports-touch/15.04/stable/hammerhead/index.json is

 {
            "description": "ubports=20181012-28857,device=20181012-23204,keyring=archive-master,tag=OTA-5,version=3",
            "files": [
                {
                    "checksum": "1308a6e7c6805789bddcd9108f6ab2b430908a3d2a0f0a0253cac0a739431ca4",
                    "order": 0,
                    "path": "/pool/ubports-1e002fd7e685421651a9b8dc15b0147c4460993169acd0519183533f17331c84.tar.xz",
                    "signature": "/pool/ubports-1e002fd7e685421651a9b8dc15b0147c4460993169acd0519183533f17331c84.tar.xz.asc",
                    "size": 397291364
                },
                {
                    "checksum": "3a01afd8a4cbdb42d90ac91dbcc7327d0bc8f6e8973b9a52f59f50a7f885c732",
                    "order": 1,
                    "path": "/pool/device-964a84f55b03eca87dacd34dffc9c61c4b50ae471c6b675eafdc2a2eacfad29c.tar.xz",
                    "signature": "/pool/device-964a84f55b03eca87dacd34dffc9c61c4b50ae471c6b675eafdc2a2eacfad29c.tar.xz.asc",
                    "size": 59352468
                },
                {
                    "checksum": "5b6e870acf8944330acb2a9dad6f5270bd06c377603a3115f805290e989ab421",
                    "order": 2,
                    "path": "/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz",
                    "signature": "/pool/keyring-4c4e7ef380ebcfa2c31084efa199138e93bfed8fc58aa3eb06bdf75a78af9b57.tar.xz.asc",
                    "size": 1504
                },
                {
                    "checksum": "977cf397b86b388539e2d2436e753e618694216a215ad0562b3adf2c8b8bf66f",
                    "order": 3,
                    "path": "/ubports-touch/16.04/stable/hammerhead/version-3.tar.xz",
                    "signature": "/ubports-touch/16.04/stable/hammerhead/version-3.tar.xz.asc",
                    "size": 456
                }
            ],
            "phased-percentage": 5,
            "type": "full",
            "version": 4,
            "version_detail": "ubports=20181012-28857,device=20181012-23204,keyring=archive-master,tag=OTA-5,version=3"
        }

There is a reference to both OTA-5 and 16.04